The present invention relates to spread spectrum signal transmission and acquisition techniques and more particularly to an adaptive acquisition technique for enabling improved code signal acquisition in multiple access code systems using code division multiplexing.
In communication systems, a group of modulation techniques has been developed which produce what is known in the prior art as spread spectrum systems. Spread spectrum techniques are generally well known and are specifically characterized by their use of large bandwidths to send small amounts of information. While there are a variety of spread spectrum techniques, all have two characteristics in common: (1) the transmitted bandwidth is much greater than the bandwidth of the information being sent; and (2) some function other than the information being transmitted is used to determine the bandwidth of the transmitted signal.
Typically, while more complex than conventionally modulated communication systems, spread spectrum systems offer several advantages that other systems do not. By way of example, a spread spectrum system can be used to selectively transmit to designated receivers within a communication system. In such instances, each of a plurality of receivers in the system are assigned a different code. A transmitter then selectively contacts any of the individual receivers by modulating the transmitted signal with the same code as that assigned to the particular receiver. The receiver in turn matches its reference code in phase with the transmitted code to allow demodulation of the transmitted information. In this manner, information can be transmitted to a given receiver while preventing that transmission to other receivers in the communication system.
Spread spectrum systems also allow code division multiplexing wherein a plurality of multiple access codes are used to enable a plurality of transmitters and receivers to operate on the same frequency. In this example, a plurality of transmitters are each assigned a different code which is used to modulate the same carrier frequency. Each of the receivers in the system may then select the transmitter from which information will be received by selecting the receiver reference code to be a replica of the code of the selected transmitter and synchronizing the reference code with the code received from the transmitter. In this manner, each of the receivers in the system may selectively listen to one particular transmitter of the plurality of transmitters in the system.
One such common spread spectrum technique is normally referred to as direct sequence modulation. In direct sequence systems, the signal transmission will give rise to crosscorrelation of the received signal when plural transmitters simultaneously transmit with different codes on the same carrier frequency. Likewise, during receipt and acquisition of a transmitted signal at a receiver, autocorrelation of the transmitted signal and receiver reference signal will produce unwanted autocorrelation sidelobes. The effect of any high degree of correlation between simultaneously transmitted codes and the receiver reference, is to increase the false alarm rate of the receiver (the number of false indications of transmitter and receiver reference code phase synchronizations).
In an effort to overcome the above spurious signal correlation, the codes are usually selected to minimize the crosscorrelation levels and the autocorrelation sidelobes. One such code selection procedure was developed by Gold and is referenced in the article entitled "Optical Binary Sequences for Spread Spectrum Multiplexing" in the I.E.E.E. Transactions on Information Theory, Volume IT-13, pages 619-621, dated October, 1967. Using the codes developed by Gold and others, the autocorrelation sidelobes and crosscorrelation levels are reduced significantly below the primary autocorrelation peak which represents the phase synchronization of the reference code with the transmitted code. Code signal acquisition is then obtained by comparing the correlation level of the received and local reference code with a threshold which isolates the primary autocorrelation peak from the crosscorrelation levels and largest autocorrelation sidelobes.
While such code selection techniques have improved signal acquisition, the same still result in a high number of false alarms in systems designed to operate over wide ranges in received signal power. Specifically, at high signa1 levels, there is a greater likelihood that one of the autocorrelation sidelobes, which is normally well below the primary autocorrelation peak, will exceed the set threshold and produce a false alarm or improper code signal acquisition. While the problem may be overcome by measuring correlation over the entire code phase uncertainty and determining at which point the correlation level is at a maximum, this requires a significant increase in detection time and thus increased receiver acquisition time. Receiver acquisition time will be even further increased when the code sequences are large. The technique is therefore not suitable for systems requiring fast acquisitions.
Other attempts to reduce false alarms at high signal levels have employed higher thresholds or sidelobe cancellation techniques such as that described in the article "Pseudorandom Code Sidelobe Canceller", International Telemetering Conference Proceedings, pp. 349-363, 1977. Again, such techniques are unacceptable since higher thresholds only improve strong signal performance at the expense of a substantial and unacceptable decrease in weak signal performance. Likewise, sidelobe cancellation techniques only provide a limited improvement in the strong signal performance. There is therefore a continuing and substantial need for systems and techniques which enable the detection and acquisition of the coded signal in multiple access communication systems using code division multiplexing which will improve strong signal performance without significant degrading of the weak signal performance.
Accordingly, the present invention has been developed to overcome the above known and similar shortcomings and to provide an adaptive acquisition technique which improves code signal acquisition in direct sequence spread spectrum systems.